It might make the difference between some one buying a monthly bus pass or deciding to ride their bike for the month.
knowing that they could defer some of their basic maintenance costs could make riding a more appealing option.
hshearer is probably on the right track; a cap on how much you could claim and confirmation from your employer that you commute by bike.
On the part of professional cyclists I was under the impression (if I am wrong please correct me) that they usually had sponsors to help cover their costs. Also I would bet that they don't ride their racing bike to work and it would be fair to give tax credits for the up keep of their commuter ride.